One example of an ovenable paperboard tray is disclosed in U.S. Pat. No. 4,026,458, issued May 31, 1977, to Morris et al. for "Deep Drawn Paperboard Container and Process for Making It", assigned in common with the present application. The disclosure of U.S. Pat. No. 4,026,458 is incorporated herein by reference.
One conventional process for forming such trays includes four stages. In the first stage, the paper is pulled from a roll, through a de-curl section and into a free loop. A feed wheel then meters the paper into the machine. The feed wheel is typically controlled by a stroke arm one end of which pivots around an eccentric shaft and the other end of which moves back and forth on an overrunning clutch attached to a shaft connected to the feed wheel. The paper is then fed in set increments into the machine, generally at about a 45.degree. angle to the horizontal. No precision-ground or precision-paralleled rolls are used in feeding the paper. As a result, precise control and adjustment of the web alignment relative to the press are not readily possible.
The paper is then scored to ensure that the paper will fold at the desired locations during the forming process. The paperboard is usually fed in with the coated side up, and the scoring blades are accordingly located on the reciprocating top platen of the scoring mechanism, while the complementary female grooves are on the stationary lower platen. After scoring, the paper is blanked. The blanks drop through the die platen and out of the die shoe into an aluminum blank transfer to be gravity-fed to the forming section.
The forming section of this type of press comprises a reciprocating platen (cam-operated, with spring return), a male die section bolted to the platen, and a female die section bolted to a bolster plate which is backed up by springs to limit the compression force. Each blank falls into the cast iron die and is positioned by means of stationary stops and guides. As the top die is lowered, a spring-loaded draw ring (sometimes a "brake pad" arrangement) comes in contact with the blank and holds it firmly while the top (male) die pushes or "draws" the paper into the lower die. The paper is held in dwell for 120.degree. of the cam cycle. The top die is then raised, and the finished product is ejected by means of a plunger or is lifted out by the lower die. The product then falls onto a belt-driven conveyor and is carried away to be stacked and packaged.
Because a certain minimum dwell time is required to ensure the blank is properly formed, the number of strokes per minute that can be performed by this type of machine is limited.
With conventional presses of the type described, a certain amount of web material is wasted as a result of misalignment of the blanks. The wastage increases with the width of the web.